[Illuminated One]

Well, iirc I had a similar problem (trying to hellpower a vampire count) and he cast hellpower without getting anything out of it and imediately cast another spell (before the enemy cast his spells), which is buggy.
I've always supposed that a b2 mage could use 2 slaves max.
I think the AI doesn't get that during spell selection tries to cast the spell, generating "x has cast Hellpower", realizes that he can't cast it and tries to cast another spell instead ("x has cast spell y").